Vibration isolation arrangement

ABSTRACT

A vibration isolation arrangement, for example for supporting a stretcher in an ambulance, comprises a lever (16) which extends between a base structure (6) and a support frame (4). The lever (16) can pivot about a fulcrum (30) and is acted upon by a spring (28). The fulcrum (30) is displaceable by an actuator (32) along a guide (34). Such displacement not only varies the stress of the spring (28), but also alters the moment arm (h) between the spring (28) and the fulcrum (30). When raising the support frame (4) to a predetermined level, the mechanism automatically adapts to the load on the support frame (4) to provide a constant natural frequency for the system, regardless of the weight of the patent being carried.

TECHNICAL FIELD

This invention relates to a vibration isolation arrangement forsupporting a mass on a structure which is subject to vibration. Thepresent invention is particularly, although not exclusively, concernedwith suspension apparatus for supporting a load in a moving vehicle,such as a stretcher carrying a sick or injured patient in an ambulance.

BACKGROUND ART

There is concern that the transport of injured patients by road can havea detrimental effect on their medical condition. To a significantextent, this detrimental effect can be attributed to the transmission ofoscillations of the vehicle to the patient. Vertical oscillations of thevehicle are experienced as front to back vibrations of a supine patient,and it is known that humans are sensitive far more to front to backvibrations than to up-and-down vibrations, such as are experienced by aseated or standing person in a vehicle.

There is therefore a need to support supine patients in ambulances in amanner which will isolate the patient from vertical vibrations of theambulance. Since, however, the natural frequency of any conventionalspring-mass system depends on the load, and consequently on the weightof the person being carried, a system giving adequate vibrationisolation for one person would not perform satisfactorily for anotherperson of different weight. Furthermore, it is desirable for the meanride level of the patient to be independent of their weight, in order toassist ambulance staff in performing any necessary treatment on thepatient.

GB1447261 discloses a suspension device for a vehicle seat. The devicecomprises two pairs of levers mounted in a scissor arrangement. Thelower ends of the levers of each pair are movable in a frame fixed tothe vehicle, while the upper ends of the levers carry the seat. Thepivots of the two pairs of levers are interconnected by a shaft whichmoves along a cam surface as the seat moves up and down. Springs act tobias the shaft towards the upper end of the cam surface, so biasing theseat upwardly. The inclination of the cam surface is adjustable so as toadjust the spring rate of the springs as applied to the seat. Thus, thesuspension device can be adapted to the weight of a person sitting inthe seat.

The device of GB1447261 is apparently adjusted by the occupant of theseat to obtain a firmer or softer ride according to his own preference.It is likely that the occupant would make repeated adjustments over aperiod of time until the subjective optimum is reached. However, such aprogressive adjustment is not normally possible, nor necessarilydesirable, in suspension devices used for supporting sick or injuredpeople in ambulances. In such cases, the need is to provide a suspensionsystem which automatically adjusts itself in dependence on the weight ofthe patient, in order to avoid, as far as possible, the transmission ofvibrations from the vehicle to the patient during travel under emergencyconditions.

DISCLOSURE OF INVENTION

According to the present invention there is provided a vibrationisolation arrangement for supporting a mass on a structure which issubject to vibration, the arrangement comprising a lever extendingbetween the structure and the mass, the lever being pivotable againstthe action of resilient means about a fulcrum which is displaceablerelatively to the lever along a predetermined path.

The predetermined path along which the fulcrum is displaceable ispreferably straight. This will give a constant natural frequency ofoscillation, regardless of load carried, provided that the resilientmeans has a linear characteristic over the full range of oscillation.

The orientation of the predetermined path is preferably such thatdisplacement of the fulcrum along the predetermined path not only variesthe moment arm between the fulcrum and the line of action of theresilient means, but also varies the stress in the resilient means. Thepredetermined path may be defined by a guide which is fixed with respectto the structure.

In a preferred embodiment, displacement of the fulcrum is achieved bymeans of an actuator, which may be an electric jack.

The present invention also provides suspension apparatus utilizing twovibration isolation arrangements as defined above. Where the suspensionarrangement is used to support a stretcher on the floor of an ambulance,the vibration isolation arrangements respectively support opposite endsof a platform for receiving the stretcher. The vibration isolationarrangements may be operated independently, in order to provide thefacility to adjust the fore and aft and/or side-to-side inclination ofthe stretcher.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial diagrammatic view of suspension apparatus forsupporting a stretcher;

FIG. 2 is a diagrammatic side view of the suspension apparatus of FIG.1; and

FIG. 3 is a diagrammatic plan view of the suspension apparatus.

MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1, the apparatus comprises a base 2 above which ispositioned a support frame 4. In use, the base 2 will be secured to thefloor of an ambulance. The base 2 comprises a mounting frame 6 and alongitudinal beam 8 which is supported above the mounting frame 6. Acolumn 10 extends upwardly from the mounting frame 6, and the supportframe 4 is suspended from a sleeve 12 which is slidable on the collar10. The sleeve 12 incorporates a pivot 14 which permits pivotal movementof the support frame 4 about the axis X--X. The axis X--X is positionedabove the support frame 4, approximately at the same vertical level asthe expected combined center of gravity of the support frame 4 and theload supported on it, namely a stretcher and its patient.

The support frame 4 is supported vertically with respect to the base 2by two similar vibration isolation arrangements. FIG. 2 shows one ofthese arrangements which comprises a cranked lever 16 which can beregarded as having a generally horizontal portion 18 and a generallyvertical portion 20. The free end of the horizontal portion 18 isprovided with a roller 22 which engages a flange 24 on the support frame4.

The free end of the vertical portion 20 carries a roller 26 whichengages a track secured to the mounting frame 6.

A spring 28 is hinged to the vertical portion of the lever 16 a shortdistance above the roller 26. The other end of the spring 28 engages anabutment which is fixed to the mounting frame 6.

A fulcrum for the lever 16 is provided by an element 30 mounted on theend of an actuator 32. The actuator 32 is an electric jack which can beextended or retracted under power, but which maintains its position whenpower is interrupted. The end of the actuator 32 away from the fulcrum30 is hinged to the mounting frame 6.

The fulcrum 30 is guided along a predetermined path by means of a guide34. The guide 34 is secured to the mounting frame 6, and provides astraight path for the fulcrum 30, which path is inclined to the verticalat an angle The fulcrum 30 engages a straight track on the verticalportion 20 of the lever 16, and it will be appreciated that the pathestablished by the guide 34 is inclined to the vertical portion 20.

FIG. 3 shows both of the vibration isolation arrangements. The rollers22 of the two arrangements engage the support frame 4 at diagonallyopposite positions. The actuators 32 of the two arrangements may beoperated independently from one another.

For operation, the mounting frame 6 is secured to the floor of anambulance. In order to receive a stretcher, the support frame 4 is fullylowered, by retracting the actuators 32, until the support frame 4 restson the beam 8. In this condition, the springs 28 are substantially fullyextended, and the fulcrum 30 is positioned towards the bottom of theguide 34. As a result, the lever 16 is pivoted to the right (as seen inFIG. 2) under the weight of the support frame 4.

A stretcher is then placed on the support frame 4, or wheeled intoposition over it. When the stretcher is in position, the jacks 32 areactuated to extend them. This causes the fulcrum to move upwards and tothe right along the guide 34, as seen in FIG. 2. Initially, thismovement will cause the springs 28 to be compressed, without the supportframe 4 and the stretcher being raised. However, it will be appreciatedthat displacement of the fulcrum 30 along the guide 34 not only stressesthe spring 28, but also increases the length h of the moment arm betweenthe spring 28 and the fulcrum 30. At a position depending on the loadsupported by the support frame 4, the moment exerted by the spring 28about the fulcrum 30 will exceed the moment exerted by the support frame4 acting on the roller 22. When this happens, the support frame 4 willbe lifted off the beam 8, and the stretcher will be supported solely bythe springs 28, acting through the levers 16. When the support frame 4has reached the required height, power to the actuators 32 isdisconnected, and the fulcrum 30 then remains fixed in position. Thesupport frame 4 is then isolated substantially from vibration of thebase 2 as the levers 16 pivot about the fulcrum 30 against the forceapplied by the springs 28.

Because the guide 34 is straight, the natural frequency of the system,with regard to oscillations in the vertical direction, is independent ofthe load carried by the support frame 4, provided that the springs 28have linear characteristics. The natural frequency of the system isdetermined by the angle and consequently the system can be adaptedreadily to the circumstances in which it will be used. When theapparatus is used in ambulances, it has been found that a naturalfrequency of approximately 0.5 Hertz provides optimum vibrationisolation during road travel.

The support frame, and stretcher carried on it, can be tilted, ifdesired, by operating the actuators 32 independently of each other tosupport the two ends of the support frame 4 at different levels.

The column 10 restrains the support frame 4 in the horizontal plane.Because the axis X--X is positioned close to the center of gravity ofthe load, vehicle acceleration in the Y--Y direction (FIG. 1) willinduce very little or no pitching of the load. Consequently, if theapparatus is mounted in the vehicle with the Y--Y axis extending in thefore and aft direction of the vehicle, braking and acceleration of thevehicle will not cause pitching of the load.

INDUSTRIAL APPLICABILITY

Although the present invention has been described by reference to oneaspect of its industrial applicability, namely to use in ambulances, inwhich the load to be supported will be a stretcher carrying a patient,it will be appreciated that a vibration isolation arrangement inaccordance with the present invention can have other industrialapplications. For example, the present invention could be applied to thetransport of delicate instruments and equipment which need to beprotected from oscillations of the vehicle in which they are carried.Furthermore, it is not essential for the base 2 to be mounted on ahorizontal structure. The apparatus could, instead, be used to support aload on a vertical surface which is subject to vibration. Thus,apparatus in accordance with the present invention could, for example,be used to support vibration sensitive instruments or apparatus on awall of a moving vehicle or other structure which is subject tovibration.

I claim:
 1. A vibration isolation arrangement for supporting a mass on astructure which is subject to vibration, the arrangement comprising alever extending between the structure and the mass, the lever beingpivotable against the action of resilient means about a fulcrum,,characterized in that fulcrum is displaceable relatively to the leveralong a predetermined path and in that displacement of the fulcrum alongthe predetermined path, in a direction to displace the lever against theaction of the resilient means, increases the distance between thefulcrum and the point at which the resilient means acts on the lever. 2.A vibration isolation arrangement for supporting a mass on a structurewhich is subject to vibration, the arrangement comprising a leverextending between the structure and the mass, the lever being pivotableagainst the action of resilient means about a fulcrum, characterized inthat fulcrum is displaceable relatively to the lever along apredetermined path and in that opposite ends of the lever are adapted toengage, respectively, the mass and the structure in a manner permittingdisplacement of the respective ends of the lever relatively to the massand the structure.
 3. In apparatus for supporting a mass on a basestructure which is subject to vibration, wherein a vibration isolationarrangement is provided between the mass and the base structure, thevibration isolation arrangement comprising:a fulcrum supported by thebase structure; a lever extending between the mass and the basestructure, the lever being pivotable about the fulcrum upon relativedisplacement between the mass and the base structure; and resilientmeans acting on the lever to oppose pivotal movement of the lever underthe load applied to the lever by the mass, wherein the fulcrum isdisplaceable relatively to the lever along a predetermined path which isfixed relatively to the base structure.
 4. Apparatus as claimed in claim3, wherein the predetermined path is straight.
 5. Apparatus as claimedin claim 3, wherein the predetermined path is defined by a guide whichis fixed with respect to the base structure.
 6. Apparatus as claimed inclaim 3, wherein an actuator is provided for displacing the fulcrumalong the predetermined path.
 7. Apparatus as claimed in claim 3,wherein the predetermined path extends obliquely from the base structurein a direction towards the mass and opposite the direction of the forceapplied to the lever by the resilient means.
 8. Apparatus as claimed inclaim 3, wherein opposite ends of the lever are adapted to engage,respectively, the mass and the base structure in a manner permittingdisplacement of the respective ends of the lever relatively to the massand the base structure.
 9. Suspension apparatus comprising a loadcarrying member which is supported on a base structure by two vibrationisolation arrangements disposed at opposite ends of the load carryingmember, each vibration isolation arrangement comprising:a leverextending between the mass and the base structure; a fulcrum supportedby the base structure, the lever being pivotable about the fulcrum uponrelative displacement between the mass and the base structure, thefulcrum being displaceable relatively to the lever along a predeterminedpath which is fixed relatively to the base structure; and resilientmeans acting on the lever to oppose pivotal movement of the lever underthe load applied to the lever by the mass.
 10. Suspension apparatus asclaimed in claim 9, further comprising retaining means for preventingrelative displacement of the load carrying member and the base structureparallel to one another.
 11. Suspension apparatus as claimed in claim10, wherein the retaining means comprises an elongate guide member whichis fixed to the base structure and on which a carrying element isslidable, the load carrying member being pivotably connected to thecarrying element.
 12. A vehicle including suspension apparatuscomprising a load carrying member which is adapted to receive astretcher and is supported on the base structure by two vibrationisolation arrangements disposed at opposite ends of the load carryingmember, each vibration isolation arrangement comprising:a leverextending between the mass and the base structure; a fulcrum supportedby the base structure, the lever being pivotable about the fulcrum uponrelative displacement between the mass the base structure, the fulcrumbeing displaceable relatively to the lever along a predetermined pathwhich is fixed relatively to the base structure; and resilient meansacting on the lever to oppose pivotal movement of the lever under theload applied to the lever by the mass.